Magnet system with superconducting field coils

ABSTRACT

In a magnet system with two coil systems of different mean diameter arranged coaxially with respect to the longitudinal axis of the system defined by the direction of the field in the homogeneity region, which are rotationally symmetrical and are arranged symmetrically about the transverse median plane of the system perpendicular to the longitudinal axis and can carry exciting currents which result in overlapping magnetic fields that compensate, at least approximately, for the dipole fields of the two coil systems, an operating mode selector switch device (22) configured as a superconducting network is provided, by means of which the magnet system (10) can also be switched, as an alternative to the operating mode which provides considerable dipole-field compensation in the outer space of the magnet system (10), to that operating mode in which the magnetic fields generated by the two coil systems (11 and 12) at the center thereof are aligned in the same direction. In a special configuration, the ratio B 0i  :B 0a  between the values of the field strengths B 0  1 and B a  generated by the inner field coil (11) and the outer field coil (12) is equal to 3.

BACKGROUND OF THE INVENTION

The invention relates to a magnet system with superconducting fieldcoils for generating a constant magnetic field, suitable for NMRexperiments, which has a high field strength in the experimental volumeand sufficient homogeneity for NMR tomographic and/or spectroscopicexperiments.

Such magnet systems, which have two coil systems of different meandiameters arranged coaxially with respect to the longitudinal axisdefined by the field direction in the homogeneity region, which arerotationally symmetrical and are arranged symmetrically about thetransverse median plane perpendicular to the longitudinal axis, andwhich can be excited with currents which generate magnetic fields thatcompensate, at least approximately, for the dipole fields of the twocoil systems in the outer space of the magnet system, are known (forexample from EP 0 138 270 A2, EP0 144 171 A1, DE 38 29 175 A1, and GB 2206 242 A).

In the magnet system disclosed by EP 0 144 171 A1, the field coils whose(dipole) fields are intended to be largely compensated for in the outerspace of the magnet system are permanently wired in series. In themagnet system disclosed by DE 38 29 175 A1, parallel wiring of the fieldcoils which develop the two oppositely aligned magnetic moments is alsodisclosed as an alternative to series connection thereof. Although theseknown systems can be operated with currents of varying intensitieswithout thereby significantly impairing the shielding effect in theouter space, the usable field strengths are nevertheless limited, by thefact that the magnetic field generated by the outer field coil is alwaysaligned opposite to the magnetic field generated by the inner fieldcoil, to a value that is less than the absolute field strength of themagnetic field generated by the inner field coil.

In contrast, the magnet systems of the aforesaid kind disclosed by EP 0138 270 A2 and GB 2 206 242 A are configured in such a way that the twosuperconducting field coils can be energized with currents of differentstrengths, with at least one of the two field coils being energizedinductively, and the two field coils each forming a self-containedcurrent circuit. Depending on the current strength value to which theshield--i.e., outer--field coil is energized, it is then possible tovary the field strength that can be utilized for an experiment between aminimum value--which results when the outer, shielding field coil isenergized to a maximum value for its operating current strength(assuming a predetermined charge of the inner field coil)--and a maximumvalue which results when the outer, shielding field coil is notenergized.

The magnetic field strengths in the experimental volume that can beutilized with known magnet systems (i.e., those having active shieldingprovided by an external field coil in the outside space) are thereforetwo low for many experimental purposes, so that the known magnet systemsappear to be suitable, at best, for tomographic experiments in which theprimary requirement is good field homogeneity in the largest possibleexperimental volume, while high field strength is not of criticalimportance. However, the known magnet systems are therefore unsuitablefor investigative purposes such as NMR spectroscopy experiments, inwhich less emphasis is placed on good homogeneity of the magnetic fieldin the largest possible experimental volume at moderate field strength,while the highest possible field strength--along with good fieldhomogeneity in a relatively small experimental volume--is important.

SUMMARY OF THE INVENTION

The object of the invention is therefore to improve a magnet system ofthe aforesaid kind in such a way that even with a simple configurationit is equally suitable both for NMR tomography and for spectrographicpurposes, such as for example scientific analysis experiments.

This object is achieved, according to the invention, by the fact that anoperating mode selector switch device is provided, by means of which themagnet system can also be switched, as an alternative to the operatingmode which provides considerable dipole-field compensation in the outerspace of the magnet system, to at least that operating mode in which themagnetic fields generated by the two coil systems at the center thereofare aligned in the same direction.

The magnet system according to the invention has the advantage, from theuser's point of view, that it offers the option of two differentutilization possibilities: on the one hand a shielding mode atrelatively low field strength but with a large homogeneity volume; andon the other hand a high-field mode which allows the use ofsubstantially higher magnetic field strengths in a relatively smallerexperimental volume but with good homogeneity; in this respect themagnet system according to the invention appears primarily suitable forresearch purposes where, in general, a wider range of variation inexperimental boundary conditions is required than in the medicaldiagnostic field, for example whole-body tomography, for which themagnet system according to the invention--when set to shielding mode--isequally well suited.

From a manufacturer's point of view, it may be regarded as a noteworthyadvantage that one and the same magnet system can be offered for acorresponding variety of applications, which can lead to both aconsiderable gain in manufacturing efficiency and to an improvement indelivery availability, together with a reduction in minimum inventories.

When the ratio between the value of the field strength of the magneticfield generated by the inner field coil of the magnet system and thevalue of the field strength of the field generated by the outer fieldcoil of the magnet system is equal to around 3:1, it is possible, withthe magnet system in high-field mode, to obtain a magnetic field that istwice as great in terms of value as in the shielding mode of the magnetsystem.

In one embodiment, two field coils are utilized with each coilgenerating fields of good homogeneity, the superposition of which,regardless of whether currents of the same or different strengths areflowing through the two field coils, again results in a magnetic fieldwhose homogeneity corresponds at least to the "lower" of the homogeneityvalues of the two field coils.

Other embodiments of the present invention are suitable for applicationin a whole-body tomograph, and generates a field strength at the centerof 1 T in shielding mode, and 2 T in high-field mode.

The series connection of the two field coils of the magnet system isadvantageous for reasons associated with the quench resistance of thesuperconducting magnet system.

The operating mode selector switch device, by means of which the magnetsystem can be set to the particular desired operating mode--shieldingmode or high-field mode--prior to creation of the superconductingcurrent (energization), can be implemented as a simple multiple-polemechanical switch, although in this case a permanent connection of thecoil system to the electrical energizing unit is necessary.

This disadvantage can easily be eliminated by using as the operatingmode selector switch device a superconducting network according to thepresent invention, which can easily be implemented. Such a network alsomeans that if one of the two field coils of the magnet system fails, forexample because of a winding breakage, the other field coil can bere-energized and used independently to generate a magnetic field with arestricted homogeneity region.

Since different forces act on the windings of the magnet systemaccording to the invention in its two possible operating modes, andthese also lead to correspondingly different, albeit only negligible,deformations of the coil elements supporting the field coils and theirwindings, which also results in differing effects on field homogeneity,it is advantageous if the magnet system is also provided with awell-equipped shim system, with which field inhomogeneities resultingfrom slight shifts in the subcoils of the field coils can be compensatedfor.

In accordance with the present invention a shim system may be providedthat is suitable in this regard, usable alternatively or in combination.

In order to achieve minimum external shielding of the magnet system forthe high-field mode, and, for example, for instances in which only oneof the two field coils is being utilized or is available for operation,an iron shield can additionally be provided. The influence of such aniron shield on field strength and homogeneity can be taken into accountfor each of the two operating modes when designing the magnet system(cf. H. Brechna, "Superconducting Magnet Systems," Technische Physik inEinzeldarstellungen, Volume 18, Springer-Verlag Berlin, Heidelberg, NewYork, L.F. Bergmann Verlag, Munich, 1973).

In the interest of compactness and also for design reasons it isespecially advantageous if such an iron shield, which can consist in aknown manner of end plates and beam-shaped shielding elements connectingthem together, is arranged so that it directly surrounds the cryostatwhich contains the magnet system.

Since an iron shield also influences the field contour inside the magnetsystem, it is especially advantageous if the iron shield, in theinterest of maintaining the best possible field homogeneity inside themagnet system, can be adapted to the various possible operating modes ofthe magnet system.

The magnet system according to the invention is especially suitable formobile systems, which are taken from hospital to hospital on a shared orloan basis, and can generate a field (i.e., be energized) at each placeafter being set to the required operating mode.

In such a configuration of the magnet system, it is advantageous if atransport container for the magnet system, or parts of the vehicle onwhich the magnet system is mounted, are configured as functionalelements of the iron shield.

BRIEF DESCRIPTION OF THE DRAWING

Further details and features of the invention will be evident from thefollowing description of a specific exemplary embodiment, with referenceto the drawing in which:

FIG. 1 shows a superconducting magnet system according to the invention,consisting of two field coils, in section along a radial planecontaining its central longitudinal axis;

FIG. 2 shows the magnet system according to FIG. 1 with asuperconducting switching network as the operating mode selector switchdevice, in a simplified, schematic block diagram depiction;

FIG. 3 shows the magnet system according to FIG. 1 with a mechanicaloperating mode selector switch device, in a depiction corresponding toFIG. 2;

FIG. 4 shows a plot of the field lines (A×R) of the magnetic fieldgenerated by the magnet system according to FIG. 1 in its high-fieldoperating mode;

FIG. 5 shows a plot of lines of equal magnetic field strength in theouter space of the magnet system according to FIG. 1, in high-fieldmode;

FIG. 6 shows a plot of lines of equal field strength of the fieldgenerated by the magnet system according to FIG. 1 in its outer space inshielding mode;

FIG. 7 shows a plot of the magnetic field lines (A×R), in shieldingmode, of the magnet system according to FIG. 1; and

FIGS. 8a and 8b show an illustration of the homogeneity region of themagnetic field at the center of the magnet system according to FIG. 1,for the high-field operating mode (FIG. 8a) and for the shieldingoperating mode (FIG. 8b); and

FIGS. 9a and 9b show the basic structure of an iron shield which can beused in conjunction with the magnet system according to FIG. 1 in itsvarious operating modes, including those in which only one field coil isenergized, and can be adapted to these operating modes, in a view fromthe end (FIG. 9a) and in a schematic, simplified, perspective view (FIG.9b).

DETAILED DESCRIPTION

The magnet system according to the invention, referred to in itsentirety as 10 and illustrated in FIG. 1, whose details should now bespecifically referred to, is a superconducting magnet system consistingof an inner field coil 11 and an outer field coil 12, which is arrangedin a cryostat 13 (indicated only schematically), in which the lowtemperature necessary for superconductivity of the two field coils 11and 12 can be maintained for long periods of time.

The structure of cryostat 13 required for this purpose--with a chamberfilled with liquid helium that houses superconducting coils 11 and 12, achamber filled with liquid nitrogen that surrounds the helium chamber onall sides, and vacuum chambers and radiation shields arranged betweenthe latter and the outer skin of the cryostat--can be assumed to be knowin the art, and is therefore not illustrated in detail.

Field coils 11 and 12 and cryostat 13 are rotationally symmetrical withrespect to their common central longitudinal axis 14, and configuredsymmetrically with reference to transverse median plane 16 of magnetsystem 10 at right angles thereto, the said plane running perpendicularto the plane of the drawing as depicted in FIG. 1.

Inner and outer field coils 11 and 12 are, each configuresindependently, configured so that they can generate, within relativelylarge central volume regions 17 and 18 characteristic of the respectivecoils 11 and 12, a largely homogeneous magnetic field of high fieldstrength aligned parallel to central axis 14 of magnet system 10, thisfield being suitable for the performance of NMR spectroscopic and NMRtomographic experiments.

The axial and radial extent of homogeneity regions 17 and 18, withinwhich the value of ratios ΔB(r,z)_(i) :B_(o) i and ΔB(r,z)_(a) :B_(o) a--namely the ratios between the deviations ΔB(r,z)i,a (where i="inner"and a ="outer") of the field strengths, and the field strengths B_(o)i,a of magnetic fields B_(i) and B_(a) generated at the center 19 of thetwo field coils 11 and 12 respectively--corresponds to no more than apredefined value of, for example, 10 ppm, are generally of differentmagnitudes if field coils 11 and 12 are configured differently. In thespecific exemplary embodiment illustrated here, however, homogeneityregions 17 and 18 of inner field coil 11 and outer field coil 12,respectively, are, considered independently, approximately of the samemagnitude, since the larger diameter of outer field coil 12approximately compensates for the lower relative homogeneity.

In coil configuration 11, 12 illustrated in FIG. 1, in which inner fieldcoil 11 is a 12th-order coil (i.e. the gradients of its field B_(i)developed according to Legendre polynomials disappear up to the 11thorder ) and outer field coil 12 is an 8th-order coil (i.e. the gradientsof its field B1 disappear up to the 7th order), there results for thetwo field coils 11 and 12 the qualitative size relationship illustratedbetween their homogeneity regions 17 and 18, which have approximatelythe shape of ellipsoids of rotation whose common rotation axis iscentral longitudinal axis 14 of the magnet system.

In the configuration of the two field coils 11 and 12 explained so far,homogeneity region 17 of inner field coil 11 is approximately identicalto homogeneity region 18 of outer field coil 12.

When the magnet system is operating normally, i.e., withoutmalfunctions, the two field coils 11 and 12 are connected in series, andthus the same current I flows through them.

They are specifically configured with relation to one another so thatthe field strength B_(o) i of the axial field generated by inner fieldcoil 11 in center 19 of magnet system 10 is three times greater, interms of value, than the field strength B_(o) a of the axial fieldgenerated by outer field coil 12 at the center 19 of magnet system 10;moreover the two field coils 11 and 12 are configured, in terms of thegeometrical arrangement of their superconducting turns, in such a waythat the field strengths of the magnetic fields generated in the outerspace of magnet system 10 by the two field coils 11 and 12, consideredindependently, whose field profiles each correspond (to a very goodapproximation) to that of a magnetic dipole, are essentially equal invalue, i.e. differ from one another in terms of value, above a minimumdistance R from center 19 of magnet system 10, by only a negligiblysmall amount ΔB(R), which depends on distance R from the center 19 ofmagnet system 10 and decreases drastically with increasing distance R.

Thus two operating modes of magnet system 10 are possible, one of whichcan be described as "shielding mode," and the other as "high-fieldmode."

In shielding mode, current passes through the two field coils 11 and 12in such a way that the magnetic fields generated by inner field coil 11and outer field coil 12 are aligned opposite to one another. Thiseffectively compensates, in the outer space of the magnet system, forthe magnetic dipole fields generated therein by field coils 11 and 12,and results in outward shielding of the magnetic field generated bymagnet system 10 as a whole. In this shielding mode of magnet system 10,although the value of the field strength of the magnetic field generatedat the center 19 from the mutual superposition of the magnetic fieldsgenerated by each of the two field coils 11 and 12 corresponds only tothe difference in the values |B0_(i) |-|B_(O) a | of the magnetic fieldsgenerated by inner and outer field coils 12, each consideredseparately--and thus, in the specific exemplary embodiment selected forexplanation, to a field strength that corresponds to only 2/3 of thefield strength of the magnetic field generated at the center 19 by innerfield coil 11 alone--a definite enlargement of homogeneity region 21occurring in shielding mode is nevertheless achieved. The shielding modeis therefore particularly favorable if magnet system 10 is to be usedfor tomographic purposes. In the high-field mode, current passes throughfield coils 11 and 12 in such a way that the individual magnetic fieldsgenerated by the two field coils 11 and 12 are aligned in the samedirection, i.e., are "additively" superimposed. The value of theresulting magnetic field strength at the center 19 of magnet system 10in this operating mode corresponds to the sum of the individual values|B₀ i |+|B₀ a | of the magnetic fields individually generated by fieldcoils 11 and 12, and is thus twice as great as in shielding mode. Incomparison with the latter, however, a diminution of the usablehomogeneity region 22 must be accepted in high-field mode. Thehigh-field mode of magnet system 10 is thus especially suitable when thesystem is used for NMR spectroscopy purposes.

In order to allow operation of magnet system 10 alternatively in the twooperating modes, provision is made for an operating mode selector switchdevice, illustrated in FIG. 2 (whose details should now be referred to)and labeled 22 in its entirety, by means of which the two field coils 11and 12 can easily be energized with the superconductivity current flowdirection required for the particular operating mode.

To simplify the depiction, the two field coils 11 and 12, which arearranged coaxially in terms of their common central longitudinal axis14, are shown side by side in FIG. 2.

Operating mode selector switch device 22 is configured, in the specificexemplary embodiment illustrated, so that energizing current isdelivered through outer field coil 12, and energizing current isreturned through inner field coil 11 connected in series with outerfield coil 12; return connection 23 of field coil 11 is electricallyconnected to the corresponding connection 24 of energizing currentsupply unit 26, via conductor element 27 which can be removed aftercoils 11 and 12 are energized.

Energizing current field connection 28 of inner field coil 11 isconnected, via superconducting energizing current paths 29 and 31, tosupply connections 32 and 33 respectively of outer field coil 12,through which, alternatively and depending on the operating mode forwhich magnet system 10 is to be energized, the energizing currentprovided by energizing unit 34 at its supply output 36 is fed intomagnet system 10. The operating mode of the magnet system--high-fieldmode or shielding mode--is "selected" by means of a selector switch 37,which can be moved from its center "off" position to a function positionI, in which energizing current supply output 36 of energizing unit 34 iselectrically connected to a supply connection 32 of outer field coil 12,shown at the bottom in FIG. 2, or to a function position II, in whichthe other supply connection 33 of outer field coil 12, shown at the topof Figure, is connected to energizing current output 36 of energizingunit 34.

Supply connections 32 and 33 of field coil 12 can each also beconnected, via superconducting operating current paths 38 and 39respectively, to return connection 23 of inner field coil 11, and areconnected when magnet system 10 is in steady-state operation.

Energizing current paths 29 and 31 and operating current paths 38 and 39can be heated, over portions of their length, by means of electricallydriven heating elements 41, and can thus be switched from thesuperconducting to the resistively conducting state, so that they can beoperated as electrical switches which switch into their "off" state byactivation of the corresponding heating element, and switch back totheir superconducting state when the heat energy is turned off.

Energizing current paths 29 and 31 and operating current paths 38 and 39form, together with heating elements 41, a superconducting switchingnetwork whose possible switching states can be controlled as required byan electronic control unit 42, as schematically indicated.

For example, magnet system 10 can be energized for its shielding mode,using operating mode selector switch device 22, as follows:

Once magnet system 10, including its superconducting network 29, 31, 38,39, 41, has been brought into the superconducting state, energizingcurrent path 31 (which connects supply connection 33 of outer field coil12 to energizing current feed connection 28 of inner field coil 11),operating current path 39 (which connects the same supply connection 33of outer field coil 12 to return connection 23 of inner field coil 11),and operating current path 38 (which connects the other supplyconnection 32 of outer field coil 12 to return connection 23 of innerfield coil 11) are heated by means of heating elements 41 associatedwith them, and thus made to block the passage of superconductivitycurrent. Initially, only energizing current path 29, which connectssupply connection 32 of outer field coil 12 to energizing current feedconnection 28 of inner field coil 11, remains superconducting. Theenergizing current coupled into magnet system 10 from energizing unit 34via selector switch 37, which during the energization phase issuccessively increased to its reference value, flows, in theenergization phase, via the return connection of inner field coil 11back to energizing unit 34. Since the two field coils 11 and 12 arewound in the same direction when viewed along their common centrallongitudinal axis 14, while because of the current flow resulting fromfunction position II or selector switch 37, and the utilization ofenergizing current path 29, energizing current passes through them inopposite directions, the fields B1 and B2 generated by them, which arerepresented by arrows 43 and 44 respectively, are aligned in oppositedirections.

Once the energizing current has reached its reference value, heating ofoperating current path 39 is discontinued; it thereupon becomessuperconducting, and connects return connection 23 of inner field coil11 to supply connection 33 of outer field coil 12, through which magnetsystem 10 had been energized for the "shielding" operating mode.Energizing unit 14 is now bypassed by this operating current path, andthe superconducting operating current circuit providing shieldingoperation of magnet system 10 is closed. As soon as this occurs, heatingof operating current path 38, which connects return connection 23 ofinner field coil 11 to supply connection 32 (not utilized duringenergization) of outer field coil 12, is discontinued, as is heating ofenergizing current path 31, which with the type of energizationexplained above was not utilized, so that these current paths alsobecome superconducting. After this, by removing conductor elements27--by which (in energization mode) fixed contact 46 of selector switch37 was connected to supply connection 33 of outer field coil 12, andreturn connection 24 of energizing unit 34 was connected to returnconnection 23 of inner field coil 11--superconducting magnet system 10can be uncoupled form the resistive power supply section.

Magnet system 10 is energized for the high-field operating mode with theselector switch in function position I: in this case, duringenergization, energizing current path 31 (which connects energizingcurrent feed connection 28 of inner field coil 11 to supply connection33 of outer field coil 12) is kept superconducting, while energizingsection 29 and operating current paths 38 and 39 are heated during theenergization phase and therefore remain in a resistive state whichblocks superconductivity current. Energizing current thus flows fromoutput 36 of energizing unit 34, through selector switch 37 (which is infunction position I), to supply connection 32 of outer field coil 12,through the latter and energizing current path 31 (which is stillsuperconducting), to energizing current feed connection 28 of innerfield coil 11, and through the latter back to energizing unit 34. Oncethe superconductivity current flowing in magnet system 10 has reachedits reference strength, heating of operating current path 38 isdiscontinued, and it thereupon goes into the superconducting state, andnow bypasses energizing unit 34 and closes the circuit within thesuperconducting system. Then heating of operating current path 39 andenergizing current path 29 is also discontinued. As soon as these twocurrent paths 39 and 29 have also become superconducting, magnet system10 can be electrically uncoupled from the power supply side by removingconductive element 27 which connects return connection 23 of inner fieldcoil 11 to return connection 24 of energizing unit 34, and conductiveelement 27 which connects supply connection 32 (used in this case) ofouter field coil 12 to fixed contact 47 of selector switch 37 during theenergization phase.

If a fault is present in outer field coil 12, magnet system 10 cancontinue to be operated with inner field coil 11, the only one whichremains functional. The latter can then be energized, with selectorswitch 37 in position II, via energizing current path 31 which is keptin the superconducting state, and can, by bringing operating currentpath 39 into its superconducting state, be short-circuited, so that asuperconductivity current circulating in inner field coil 11 can beachieved. In the aforesaid case of a malfunction in outer field coil 12,it is also possible, with selector switch 37 in position I, to energizeinner field coil 11 via energizing current path 29 (which is in thesuperconducting state), and to short-circuit it by switching operatingcurrent path 38 to its superconducting state.

If, on the other hand, a fault occurs in inner field coil 11, magnetsystem 10 can also be operated with its outer field coil 12 only. Thelatter can then be energized, with selector switch 37 in position II,via operating current path 38, which is kept in the superconductingstate, and can be short-circuited by means of operating current path 39,which is blocked during energization and then switched to itssuperconducting state. Outer field coil 12 can also be energized withselector switch 37 in position I, in this case by means of operatingcurrent path 39 (which is in the superconducting state), and can, bymeans of operating current path 38, which is kept in the blocked stateduring energization, be short-circuited by switching the latter pathinto its superconducting state.

It is obvious that a superconducting magnet system with the functionalcharacteristics explained above can also be implemented in such a waythat the outer and inner field coils are wound in opposite directions.Assuming such a configuration of the magnet system, energization thereofas explained above for the high-field mode will result in current flowsuitable for the shielding mode of the magnet system, while energizationas explained above for the "shielding" operating mode will result in acurrent flow suitable for the "high-field" operating mode.

Selector switch 37 with the specific function explained above is notnecessary when the polarity of the fields generated by the two fieldcoils 11 and 12 is of no importance, i.e., when it is simply necessaryto guarantee that these fields have opposite polarity in the shieldingmode of magnet system 10, and the same polarity in its high-field mode.

Magnet system 10 illustrated in FIG. 3, whose details should now bereferred to, differs from the one in FIG. 2 in design terms only in theconfiguration of operating mode selector switch device 22' (which isconfigured here as either a simple electrically controlled changeoverrelay, or a simple manually actuated multiple-contact switch withmechanically movable switch contacts 48 and 49) and in functional termsby the fact that energizing unit 34 must remain connected to magnetsystem 10 even while the latter is in its investigative or experimentalmode.

Switch 22' can be changed over between two function positions I and II;function position II, with switch contacts 48 and 49 shown in solidlines, is associated with the "high-field" operating mode, whilefunction position I, with switch contacts 48 and 49 shown in dashedlines, is associated with the "shielding" operating mode of magnetsystem 10.

The configuration of operating mode selector switch device 22' depictedin FIG. 3 is suitable both for a superconducting magnet system 10 andfor a magnet system with resistive field coils 11 and 12.

The directions of the B_(a) fields generated by outer field coil 12associated with the various operating modes I and II are correspondinglylabeled I and II.

To explain a specific configuration of coil system 10, reference withnow be made again to the relevant details of FIG. 1, which reproducesthe configuration of inner and outer field coils 11 and 12 in a scalelongitudinal section depiction.

Inner field coil 11 consists of a total of six subcoils 61-66, which,each considered separately, extend over only a fraction of the totalaxial length L₁ of inner field coil 11, and are electrically connectedin series in the order of their reference numbers.

These subcoils 61 and 66 have a rectangular cross section when viewed inthe section plane of FIG. 1. Subcoils 61 and 66 of inner field coil 11each have the same internal diameter d₁.

The two outer field windings 61 and 66 of inner field coil 11 have anoutside diameter da1, which is somewhat greater than the outsidediameter of the innermost subcoils 63 and 64 and that of the middlesubcoils 62 and 65 located between them and the outer field windings 61and 66, with these middle subcoils 62 and 65 and inner subcoils 63 and64 having the same outside diameter d_(a2).

The inner ends 67 of inner subcoils 62 and 65 of inner field coil 11 areat an axial distance a₃ from its plane of symmetry, and their outer ends71 are at an axial distance a₄.

The inner ends 72 of outer subcoils 61 and 66 are at an axial distancea5 from symmetry plane 16 of magnet system 10. The axial distance a₆ ofouter end 73 of outer subcoils 61 and 62 of inner field coil 11corresponds to half its length L₁ /2.

Outer field coil 12 consists of a total of four subcoils 74 to 77, whichare electrically connected in series in order of their reference numbers74, 75, 76 and 77, each extend over only a portion of the total lengthLa of outer field coil 12, and, in the representation of FIG. 1, have arectangular cross section.

The axial distance between inner ends 78 of inner subcoils 75 and 76 andsymmetry plane 16 of magnet system 10 is equal to a₇.

The axial distance between outer end 79 of inner subcoils 75 and 76 andthe central symmetry plane 16 of magnet system 10 is equal to a₈.

The axial distance between inner ends 81 of outer subcoils 74 and 77 ofouter coil 12 and its symmetry plane 16 is equal to a₉. The axialdistance a10 between outer end 82 of outer subcoils 74 and 77 of outerfield coil 12 and its central symmetry plane 16 corresponds to half thevalue L_(a) /2 of the total length L_(a) of outer field coil 12.

Outer subcoils 74 and 77 of outer field coil 12 each have 2,832 turns.Inner subcoils 75 and 76 of outer field coil 12 each have 1,056 turns.These turns of outer and inner subcoils 74, 77 and 75, 76 of outer fieldcoil 12 are each arranged in 16 radially superimposed layers of turns.

Outer subcoils 61 and 66 of inner field coil 11 each have 5,304 turns,which are arranged in 34 radially superimposed layers of turns. Middlesubcoils 62 and 65 of inner field coil 11 each have 1,960 turns, whichare arranged in 28 radially superimposed layers of turns.

Inner subcoils 63 and 64 of inner field coil 11 each have 1,456 turns,which are also arranged in 28 radially superimposed layers of turns.

Subcoils 61 to 66 of inner field coil 11 and subcoils 74 and 77 of outerfield coil 12, all of which are electrically connected to one another inseries, are configured so that when magnet system 10 is in operation,the same current density I/cm² results for both field coils 11 and 12.

Subcoils 61 to 66 of inner field coil 11 and subcoils 74 to 77 of outerfield coil 12 are each wound onto a coil support 51 and 52 with a basiccylindrical tubular shape. Coil supports 51 and 52 are provided on theirouter surfaces with radial annular flanges 53 and 54, and 55,respectively, which form, in pairs, the axial boundaries of coil boxesin which subcoils 61 to 66 of inner field coil 11, and subcoils 74 to 77of outer field coil 12, are retained in an axially immovable manner andradially supported on the inside. The two essentially tubular coilsupports 51 and 52 are permanently mechanically connected to one anotherat their ends by essentially annular end plates 56 and 57. Coil supports51 and 52 and end plates 56 and 57 are usually made of aluminum. Thesupport and retaining structures by which magnet system 10 ispermanently mounted inside cryostat 13 are not shown, in the interest ofsimplified depiction. In a specific embodiment of coil system 10 withinner field coil 11 as a 12th-order coil and outer field coil 12 as an8th-order coil, their geometrical parameters a₁ to a₁₀, d_(a1) andd_(a2), and D_(i) and D_(a), have the following values:

a₁ : 6.43 cm

a₂ : 14.24 cm

a₃ : 28.99 cm

a₄ : 39.57 cm

a₅ : 66.13 cm

a₆ : 89.50 cm

a₇ : 17.77 cm

a₈ : 25.65 cm

a₉ : 73.50 cm

a₁₀ : 100.00 cm

d_(i) : 61.43 cm

d_(a1) : 66.53 cm

d_(a2) : 65.63 cm

D_(i) : 91.93 cm

D_(a) : 94.33 cm

Magnet system 10 with this geometrical configuration of its field coils11 and 12 is operated, in a typical application, with a current densityof 8,333 A/cm². With the turns and layers of turns indicated above, theresulting operating current strength for magnet system 10 is 187.5 A.

Under these operating conditions, magnet system 10 generates a B fieldwith the B field line profile 83 illustrated qualitatively in FIG. 4,whose details should now be referred to, the profile of which in theouter space of magnet system 10 corresponds (to a first approximation)to that of a magnetic dipole field, while in the inner space of magnetsystem 10, i.e., inside length L_(i) of inner field winding 11, thefield lines run approximately parallel to central longitudinal axis 14of magnet system 10. To simplify the depiction, FIG. 4 illustrates onlythe "1st" quadrant of the field profile (A×R) in a plane containingcentral axis 14 of magnet system 10 and extending at right angles tosymmetry plane 16 of magnet system 10, with the axial and radial extentof the region illustrated measuring in each case 16 m from the center 19of magnet system 10.

With magnet system 10 in its high-field mode, FIG. 5, whose detailsshould now also be referred to, illustrates the profile of the equalfield strength lines 84 ("equifield lines") that results in the outerspace of magnet system 10, which approximately corresponds to thecontours of ellipses that intersect symmetry plane 16 of magnet system10, and its central axis 14, in each case at right angles. Asillustrated to scale in FIG. 5, equifield line 84₅ corresponding to afield strength of 5 mT intersects symmetry plane 16 of magnet system 10at a radial distance of 5.4 m measured from the center 19 of magnetsystem 10, and central axis 14 of magnet system 10 at an axial distanceof 6.8 [m]. Equifield line 84₀.5 corresponding to 1/10 of this fieldstrength intersects symmetry plane 16 of magnet system 10 at a radialdistance of 12 m, and central longitudinal axis 14 of magnet system 10at an axial distance of 14.5 m.

FIG. 6, whose details should now be referred to, shows--in a depictionwhich corresponds to the depiction in FIG. 5, including scale--theprofile of equifield lines 86 for the shielding mode of magnet system10, which results in a field strength of 1 Tesla at the center 19 ofmagnet system 10.

In this case, the 5 mT equifield line 86₅ intersects symmetry plane 16of magnet system 10 at a radial distance of approximately 2.3 m from thecenter 19 of magnet system 10, and central longitudinal axis 14 at adistance of 2.5 m; while field line 86₀.5, corresponding to a fieldstrength of 0.5 mT, intersects symmetry plane 16 of magnet system 10 ata radial distance of 3.9 m and central longitudinal axis 14 of magnetsystem 10 at an axial distance of 4.4 m, measured in each case from thecenter 19. Inside an angular region α of approximately 30 degrees,(which is sector-shaped when viewed from the center 19 of magnetic field10) which lies between 25 degrees and 55 degrees when measured fromcentral longitudinal axis 24, equifield lines 86 have a profilewhich--when viewed in the plane of the illustration--bends in towardsthe center 19 of magnet system 10 and substantially departs from theelliptical, with the region within which the field strength of themagnetic field drops to a value of 0.5 mT being substantially smaller(i.e., by a factor of 1/3) than in the high-field operating mode ofmagnet system 10.

The reason is that with magnet system 10 in shielding mode, the magneticfield generated by the latter--the field line profile (A ×R) of which isreproduced in FIG. 7 (whose details should now be referred to) in adepiction corresponding to FIG. 4 but at a scale enlarged by a factor offour--is largely enclosed "inside", i.e., "between" inner field coil 11and outer field coil 12, and leakage fields can emerge to a much lesserextent into the outer space of magnet system 10.

When reference numbers are indicated in FIGS. 4 to 7 and are notspecifically mentioned in the relevant portions of the description, theysignify a reference to the description of the previous, identicallyreferenced elements of magnet system 10.

FIGS. 8a and 8b, whose details should now be referred to, depict forhigh-field mode (FIG. 8a) and shielding mode (FIG. 8b) of magnet system10, plots for the deviation ΔB=B-B₀ of the field strength B of the Bfield generated by magnet system 10 from the value B₀ of the fieldstrength at the center 19 of magnet system 10, as a function of distanceZ from the center 19, measured along central axis 14.

As is immediately evident from a comparison of FIGS. 8a and 8b, theaxial region within which field strength B differs by less than 10 ppmfrom the field strength B₀ prevailing at the center - i.e., by less than2.sup.. 10⁻⁵ T for the high-field mode, and by less than 10⁻⁵ T for theshielding mode (B₀ =1 T)--is different, being substantially (i.e. morethan 20%) greater in the shielding mode than in the high-field mode ofmagnet system 10.

This difference in homogeneity results from the different 8th-ordercoefficient of outer field coil 12, which is negative. In shieldingmode, in which outer field coil 12 carries negative current, the8th-order contribution leads to an advantageous enlargement of thehomogeneity region along the axis of rotation, and in the plane ofreflection 16, perpendicular thereto, in the radial direction.

Lastly, FIGS. 9a and 9b, whose details should be expressly referred to,explain simple embodiment possibilities of an iron shield, labeled 87 inits entirety, which for example can provide shielding of the magneticfield that would otherwise be generated in the outer space by magnetsystem 10, even when magnet system 10 is being used in high-field mode.The same applies, analogously, when magnet system 10 is being operatedwith only one of its field coils 11 or 12.

This shield 87 has fourfold symmetry in terms of its centrallongitudinal axis 14, which in the operating position of shield 87coincides with central axis 14 of magnet system 10. Shield 87 consistsof end plates 88 and 89 running perpendicular to central longitudinalaxis 14, and beam-shaped iron shielding elements 91 and 92 connectingthe said plates together and running parallel to central longitudinalaxis 14; the said elements are connected together in a removably solidmanner by threaded connectors (not shown). End plates 88 and 89 havecircular central openings 93, the diameter of which corresponds to thediameter of central hole 94 of cryostat 13 which contains magnet system10. Beam-shaped shielding elements 91 and 92 are configured as ironplates with a trapezoidal cross section, the lateral flank surfaces 95and 96 of which run at 45 degrees to base surfaces 97, 98, and 99, 101,respectively, of the inner beam-shaped shielding elements 91 and outerbeam-shaped shielding elements 92, as depicted in FIGS. 9a and 9b;beam-shaped shielding elements 91 and 92 are arranged so that theirlongitudinal median planes 102 and 103 which run in the direction ofcentral longitudinal axis 14 and intersect along the latter enclose ineach case an angle of 45 degrees with the vertical longitudinal medianplane 104 of the overall system comprising magnet system 10 and ironshield 87.

Iron shield 87 is configured so that end plates 88 and 89 can be placedin the immediate vicinity of the annular end surfaces of the casing ofcryostat, and possibly even in contact with it, and also so that theradial distance between inner beam-shaped shielding element 91 and theouter casing surface of the cryostat casing is small, i.e., a fewcentimeters, so that iron shield 87 directly surrounds cryostat 13 whenviewed in section in the latter's circumferential direction.

By appropriately taking into account the effect of iron shield 87 on thefield profile inside magnet system 10, it is possible to achieveimproved field homogeneity when the latter is in high-field mode. Thesame applies, analogously, to the case in which the magnet system isoperated with only one of its two field coils 11 or 12, although thenthe iron shield must be adaptable to the particular mode in order toachieve optimum field homogeneity.

Such adaptation possibilities for iron shield 87 can involve, forexample, the fact that the outer beam-shaped shielding elements 92 canbe removed, and/or that the effective thickness of end plates 88 and 89can be varied by removing or applying an additional end plate, asindicated schematically in the left part of FIG. 9b by a two-part endplate 89. It is evident that such end plate elements do not necessarily,as shown in FIG. 9b, need to have the same plate surface area andperipheral contour, but may possibly also be configured as round disks,the outside diameter of which is considerably smaller than the diameter,measured between parallel contour edges, of the end plates illustrated.

It is evident that the possibilities illustrated for configuring an ironshield 87 that can be used in conjunction with magnet system 10 cannotbe exhaustive, but rather that it is entirely possible for one skilledin the art, having understood the aforesaid basic principle of avariable and adjustable iron shield, to determine its appropriateconfiguration by means of calculation and/or experiment.

In a specific configuration not illustrated in the interest ofsimplicity, magnet system 10, possibly including an iron shield 87 asexplained above, is embodied as a transportable unit, which can betransported by means of a vehicle to the particular utilizationlocation, for example to a hospital where magnet system 10 is usedprimarily for tomographic purposes, or to a research institution whereit can be used principally for spectroscopic purposes.

In the specific embodiment of such a transportable magnet system 10, itis advantageous if the latter, including the energizing unit, ispermanently mounted on the transport vehicle, with parts of thetransport vehicle being utilized as functional elements of the magnetsystem's iron Shield. In such an embodiment of magnet system 10 allowingversatile use, the latter is suitable as a shared or loaned system,which permits very economical operation both for the operator and forthe user.

What is claimed is:
 1. Magnet system for generating a magnetic fieldsuitable for NMR experiments, having a high field strength in anexperimental volume and sufficient homogeneity for NMR experiments,tomographic, and/or spectroscopic investigations, said magnet systemcomprising:inner and outer superconducting field coil systems ofdifferent mean diameter arranged coaxially with respect to alongitudinal axis of the magnet system defined by a direction of ahomogeneity region, said coil system being rotationally symmetrical andarranged symmetrically about a transverse median plane of the magnetsystem perpendicular to the longitudinal axis, said coil systems beingcapable of carrying exciting currents for producing over-lappingmagnetic fields that compensate, at least approximately, for dipolefields of the two coil systems; and operating mode selector switch meansfor controlling current in the two coil systems so that in one operatingmode dipole-field compensation in an outer space of the magnet system isprovided and in another operating mode magnetic fields generated by thetwo coil systems, at the center thereof, are aligned in one direction.2. Magnet system according to claim 1, wherein a ratio B_(o1) :B_(oa)between values of field strengths B_(ol) and B_(oa) generatedrespectively by the inner field coil system and and the outer field coilsystem is equal to approximately
 3. 3. Magnet system according to claim1 wherein the inner field coil system is between an 8th-order coil and a12th-order coil; and the outer field coil system is between a 6th-ordercoil and an 8th-order coil.
 4. Magnet system according to claim 3,wherein the outer field coil system comprises at least four subcoils ofthe same winding density, arranged at axial distances from one anotherand through which, when viewed in a direction of a central axis of themagnet system exciting current flows in the same direction, saidsubcoils having same inside diameter D₁ and a same outside diameterD_(a), with a number of turns in outer outer field subcoils beingbetween 2.5 and 3.5 times greater than a number of turns in inner outerfield subcoils of the subcoils, a radial thickness (D_(a) -D₁) of theouter field subcoils being small compared to the mean diameter, (D_(a)+D₁)/2, thereof.
 5. Magnet system according to claim 3 wherein the innerfield coil system comprises at least six subcoils of the same windingdensity, arranged at axial distances from one another and through which,when viewed in a direction of a central axis of the magnet system,exciting current flows in the same direction said subcoils having a sameinside diameter d₁, with a number of turns in middle inner fieldsubcoils located between respective inner inner field subcoils and outerinner field subcoils, being between 1.3 and 1.6 times greater than anumber in the inner inner field subcoils, and a number of turns in theouter inner field subcoils being between 3.5 and 4 times greater thanthe number in the inner, inner field subcoils, a radial thicknesses(d_(a2) -d_(a1)) of the inner and middle inner field subcoils and aradial thicknesses (d_(a1) -d₁) of the outer inner field subcoils beingsmall compared to the inside diameter, d₁.
 6. Magnet system according toclaim 4 wherein the inner field coil comprises at least six subcoils ofthe same winding density, arranged at axial distances from one anotherand through which, when viewed in the direction of the central axis ofthe magnet system, exciting current flows in the same direction, theinner field coils subcoils having a same inside diameter d₁, with anumber of turns in middle inner field subcoils located betweenrespective inner inner field subcoils and outer inner field subcoilsbeing between 1.3 and 1.6 times greater than a number in the inner innerfield subcoils, and the number of turns in the outer inner fieldsubcoils being between 3.5 and 4 times greater than the number in theinner inner field subcoils, a radial thicknesses (d_(a2) -d_(a1)) of theinner and middle inner field subcoils and a radial thicknesses (da_(a)-d₁) of the outer inner field subcoils being between 4 and 6%, and 5 and7%, respectively, of the inside diameter (d₁) a length L₁ of the innerfield coil, measured between outer ends of the outer inner fieldsubcoils, is between 85 and 95% of a correspondingly measured lengthL_(a), measured between outer ends of the outer outer field subcoils, ofthe outer field coil, an inside diameter D₁ of the outer field coilcorresponds to between 90 and 95% of the axial length L_(a) of the outerfield coil, and a ratio D₁ /d₁ between the inside diameter Di of theouter field coil and the inside diameter d₁ of the inner field coil isequal to approximately 1.4.
 7. Magnet system according to claim 1,wherein the two field coil systems are connected in series.
 8. Magnetsystem according to claim 1, wherein the switch means comprises asuperconducting switching network.
 9. Magnet system according to claim 8wherein the switching network comprises four superconducting electricalcurrent paths, which can be switched, by heating, into a resistive statefor blocking superconductivity current, such that each of twoconnections of the outer field coil system can be connected to andblocked off from each of two connections of the inner field coil system.10. Magnet system according to claim 9 wherein the switching network isconfigured so that during energization, only one of the twosuperconducting current paths, by means of which the two field coils areconnected in series with one another, is open in each case, and, as soonas the reference current density is reached, the current path whichbypasses the energizing unit and closes the circuit to the field coils,connected in series, is switched into the superconducting state, andthereafter the other two current paths are also switched tosuperconducting transmission.
 11. Magnet system according to claim 1further comprising shim system means for generating a field gradient inorder that in an alternative operating mode, resulting in homogeneitiesof the magnetic field in an experimental volume can be at leastpartially compensated for.
 12. Magnet system according to claim 1,further comprising a passive iron shield disposed around the inner andouter field coil systems, for preventing magnetic field generationoutside of the magnetic system.
 13. Magnet system according to claim 12,wherein the iron shield comprises end plates and beam-shaped elementsaxially connecting the said plates to one another, said iron shieldsurrounding a cryostat of the magnet system.
 14. Magnet system accordingto claim 1 wherein the two field coil systems are connected in seriesand the switch means comprises a superconducting network.
 15. Magnetsystem according to claim 14 wherein the inner field coil system isbetween an 8th-order coil and a 12th-order coil; and the outer fieldcoil system is between a 6th-order coil and an 8th-order coil. 16.Magnet system according to claim 15 wherein the inner field coil systemcomprises at least six subcoils of the same winding density, arranged ataxial distances from one another and through which, when viewed in adirection of a central axis of the magnet system, exciting current flowsin the same direction, said subcoils having a same inside diameter d₁,with a number of turns in middle inner field subcoils located betweenrespective inner inner field subcoils and outer inner field subcoils,being between 1.3 and 1.6 times greater than a number in the inner innerfield subcoils, and a number of turns in the outer inner field subcoilsbeing between 3.5 and 4 times greater than the number in the inner innerfield subcoils, a radial thicknesses (d_(a2) -d_(a1)) of the inner andmiddle inner field subcoils and a radial thicknesses (d_(a1) -d₁) of theouter inner field subcoils being small compared to the inside diameter,d₁.
 17. Magnet system according to claim 15, wherein the outer fieldcoil system comprises at least four subcoils of the same windingdensity, arranged at axial distances from one another and through which,when viewed in a direction of a central axis of the magnet system,exciting current flows in the same direction, said subcoils having sameinside diameter D₁ and a same outside diameter D_(a), with a number ofturns in outer outer field subcoils being between 2.5 and 3.5 timesgreater than a number of turns in inner outer field subcoils of thesubcoils, a radial thickness (D_(a) -D₁) of the outer field subcoilsbeing small compared to the mean diameter, (D_(a) +D₁)/2, thereof. 18.Magnet system according to claim 17 wherein the inner field coilcomprises at least six subcoils the same winding density, arranged ataxial distances from one another and through which, when viewed in thedirection of the central axis of the magnet system, exciting currentflows in the same direction, the inner field coils subcoils having asame inside diameter d₁, with a number of turns in middle inner fieldsubcoils located between respective inner inner field subcoils and otherinner field subcoils being between 1.3 and 1.6 times greater than anumber in the inner inner field subcoils, and the number of turns in theouter inner field subcoils being between 3.5 and 4 times greater thanthe number in the inner inner field subcoils, a radial thicknesses(d_(a2) -d_(a1)) of the inner and middle inner field subcoils and aradial thicknesses (da_(a) -d₁) of the outer inner field subcoils beingbetween 4 and 6%, and 5 and 7%, respectively, of the inside diameter(d₁) a length L₁ of the inner field coil, measured between outer ends ofthe outer inner field subcoils, is between 85 and 95% of acorrespondingly measured length L_(a), measured between outer ends ofthe outer outer field subcoils, of the outer field coil, an insidediameter D₁ of the outer field coil corresponds to between 90 and 95% ofthe axial length L_(a) of the outer field coil, and a ratio D₁ /d₁between the inside diameter D₁ of the outer field coil and the insidediameter d₁ of the inner field coil is equal to approximately 1.4. 19.Magnet system according to claim 14 wherein the switching networkcomprises four superconducting electrical current paths, which can beswitched, by heating, into a resistive state for blockingsuperconductivity current, such that each of two connections of theouter field coil system can be connected to and blocked off from each oftwo connections of the inner field coil system.
 20. Magnet systemaccording to claim 19 wherein the switching network is configured sothat during energization, only one of the two superconducting currentpaths, by means of which the two field coils are connected in serieswith one another, is open in each case, and, as soon as the referencecurrent density is reached, the current path which bypasses theenergizing unit and closes the circuit to the field coils, connected inseries, is switched into the superconducting state, and thereafter theother two current paths are also switched to superconductingtransmission.